A single heavy stakeholder can't upvote it to the top of the list against the wishes of other stakeholders unless that stakeholder has more stake than all other active stakeholders combined.
Based on witness voting statistics, the only possible stakeholder that could do this given the current distribution is Steemit Inc, and I'm not sure even they could do it if there was a truly concerted disagreement on the part of all other active voters.
Also, when I spoke to Ned, he assured me that Steemit would not vote on worker proposals and while Steemit could always renege on that, I think the resulting damage to trust would never make it worthwhile to do so.
but what if 3 big stakeholders make a deal or 5 of them? i know that 1 vote for 1 acc would not be fair for all the people that invested a lot but this also has a lot of space for abuse. as someone who's vote means nothing i wish you all the luck.
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The potential downside to such behavior is a reputational loss to the system which affects the value of their Steem holdings. In practice, I haven't seen this abuse vector happen in BitShares.